In many laser applications, accurate and precise knowledge of the laser emission wavelength is required. For example, wavelength division multiplexing for optical telecommunications typically requires a wavelength precision on the order of several GHz. Spectroscopic applications, such as cavity ring-down spectroscopy, can have much more stringent wavelength precision requirements (e.g., on the order of 1 MHz). Although lasers inherently having a very precisely defined output wavelength are known, such lasers tend to be costly and tend to operate at a fixed wavelength, both of which are significant practical disadvantages. Instead, it is usually preferred to employ small, inexpensive lasers, such as semiconductor diode lasers, whenever possible. However, the output wavelength of a diode laser is typically not known with sufficient accuracy for spectroscopic applications, even if a wavelength calibration is carried out on a laser diode (e.g., wavelength vs. diode current).
Accordingly, various approaches have been considered in the art for providing wavelength monitoring, especially in connection with laser diodes. One conceptually straightforward approach is to provide the laser diode output to a general purpose spectrometer having sufficient accuracy and precision to meet the overall requirements. Since general purpose spectrometers having a resolution on the order of 1 MHz for optical wavelengths are large, expensive laboratory instruments, this approach is typically too expensive to consider seriously. Instead, the various approaches that have been considered can be regarded as providing spectrometers having sufficient accuracy and precision that are not “general purpose”, thereby allowing exploitation of special features of the wavelength monitoring problem.
Some such approaches are based on the use of an etalon as a wavelength sensitive element. An etalon provides a response (e.g., reflectance or transmittance) that is a periodic function of wavelength. The period of an etalon is referred to as its free spectral range (FSR). Measurement of the etalon response provides an ambiguous wavelength measurement, since two wavelengths in each FSR are compatible with the measured etalon response. Removal of this ambiguity can be performed in various ways. For example, a separate wavelength measurement having a precision sufficient to resolve wavelengths separated by 1 FSR can be employed to remove the ambiguity. Alternatively, the laser output wavelength may be known as a function of its operation condition (e.g., current) with sufficient precision to remove the ambiguity. Thus ambiguity removal can be regarded as providing a coarse wavelength measurement having a resolution on the order of the FSR, while the etalon response provides a fine wavelength measurement having a resolution much less than the FSR.
A key advantage of etalon-based approaches is reduced size and cost compared to many other approaches. For example, a 2 mm thick etalon of BK7 glass has an FSR of about 50 GHz, and can provide wavelength measurement precision on the order of 1 MHz.
However, etalon based wavelength monitors having only a single etalon have “dead spots” in their performance, where sensitivity to changes in wavelength is greatly, and undesirably, reduced. These dead spots arise from the local maxima and local minima of the etalon response, where each FSR includes at least one local maximum and at least one local minimum of the etalon response. The problem of dead spots in an etalon wavelength monitor can be alleviated by providing a second etalon in the wavelength monitor. For example, etalons having two different FSRs are considered in U.S. Pat. No. 5,798,859. Another example is U.S. Pat. No. 6,178,002, where two etalons having the same FSR but having a relative phase shift of about 90° are considered. In these examples, a wavelength signal is derived from two etalon responses.
Combination of two etalon responses to provide a single wavelength signal is a problem which arises specifically in connection with dual-etalon wavelength monitors. Such combination is especially critical in cases where the wavelength signal is employed for closed loop control of the laser wavelength. Although closed loop wavelength control based on signals from two etalons is considered in U.S. Pat. No. 6,178,002, this loop includes relatively complicated calculations.
Accordingly, it would be an advance in the art to provide dual etalon wavelength monitoring where etalon signals are combined to provide a wavelength signal in a simpler manner.